This section provides background information related to the present disclosure which is not necessarily prior art.
A typical high power UPS 100, such as shown in FIG. 1, typically has a fully rated isolated input transformer 102 having a primary to which a power feed from a utility substation is coupled. The input transformer 102 has a secondary for each rectifier 104 of the UPS. Each secondary is coupled to an input of a respective rectifier 104 and an output of each rectifier 104 is coupled to an input of an inverter 106. There may be a single inverter to which all the rectifiers are coupled, or there may be an inverter for each rectifier. The back-up power source 108, such as a battery bank, is also typically coupled to the input of the inverter(s) 106. An output of the inverter(s) 106 is coupled to a primary of a fully rated output transformer 110, and the secondary of the output transformer 110 is coupled to the power distribution system 112 of the facility. The output transformer 110 typically boosts the voltage from the inverter 106 to the desired output voltage, such as 480 VAC. A bypass switch 114, when closed, bypasses the UPS and connects the facility's power distribution system directly to a bypass source, such as to the power feed from the utility substation.
In order to control the DC voltage in a phase controlled inverter, such as a rectifier of a UPS, the DC voltage must be measured as a feedback signal. If this measurement fails (e.g., open circuited or shorted circuited), the controller will “choose” the wrong firing angle. Hence, the DC voltage will decrease rapidly to zero or increase rapidly potentially causing damage to the components. In an uninterruptible power supply (“UPS”) application, the critical bus (the output bus of the UPS) would be forced onto battery until that energy source is depleted at which time the critical bus will be transferred to the unconditioned, unpredictable utility source.
The existing firing angle control of the rectifier of a UPS uses a Proportional+Integral (PI) controller. The transient response of this controller allows for large over and under shoots when the load changes (without a battery source connected.). When a big load hit happens, DC bus voltage will decrease severely and may cause DC bus under-voltage. When a load dump happens, DC bus will increase rapidly and may cause DC bus over-voltage. Both cases will shut down the rectifier.
In UPS applications, some rectifier faults (e.g. SCR short, SCR drive board fail, etc.) are difficult to detect due to limited measurements that are available.